1. Field of the Invention
The present invention relates generally to an under-reaming pile bore excavator and the method of its operation, and more particularly to an excavator such that an under-reamed part of a pile bore is excavated using a plurality of slidable wing bits moving downward and extending outward at the bottom of a straight pile bore along a plurality of guide rails by applying a relatively small torque thereto.
2. Description of the Prior Art
As an economical method of executing pile foundations, an under-reamed bore foundation is well known in which a larger diameter bore is excavated at the bottom of a straight pile bore to increase the end bearing capacity against a vertical load applied to the pile.
Conventionally, the method of reverse circulation drill excavation has been used for executing these under-reamed pile foundations. In this method, however, since the under-reamed bore is excavated by extending the extendable wing bits horizontally by hydraulic cylinders without moving the wing bits downward, it is very difficult to form a stable impermeable surface layer preventing water from penetrating into the soil wall of the excavated bore and preventing lumps of soil from being dislodged therefrom, and additionally a relatively great torque is required to excavate the under-reamed part of the bore.
A more detailed description of the prior-art under-reaming bore excavator will be made hereinafter under DETAILED DESCRIPTION OF THE INVENTION with reference to the attached drawings.